I wrote this after fiveoh's first reply so it might be repeating what others have said, but 'll post anyway.
So you would end up working 8 extra years if you worked til 48?
1. You need to give her a lot of time. Most women haven't had to sort out their feelings about being a main breadwinner vs. having a working partner.
2. I would combine what Rural said with what I said about being a house husband. Everybody is mentioning chores around the house and you haven't talked about it, so that suggests it's not the direction you're thinking. Whatever you're thinking your contribution is going to be, I would start doing it now. Don't just make a list of what you could do: do it. If it's something that you could do part-time now, like being with your kids or volunteering, really throw yourself into it. Become an indispensable coordinator at your church's youth program. Be practically too busy to have your regular job. Otherwise it's just going to be too difficult for her to imagine.
It's sexist, but we tend to assume that women are naturally good at this stay-at-home spouse stuff like childcare and community work whereas we assume that men are not good at being supportive. So you need to offer a demonstration. Also sexist, but some men tend to get a boost from being the sole breadwinner, whereas society tends to see a women supporting a family as being an indentured loser (oddly, it sees stay at home wives the same way. it is sort of hard to win.). And men's sexual attractiveness tends to plummet if they're not earning money or in a position of respect. The whole thing is clearly not about whether she's willing to work til 50 on her own behalf, but the dynamic of the relationship with you, so you need to be focusing on that. Make absolutely sure you understand her misgivings in detail. Is it really about buying more things? Or is it the potential for a different lifestyle? Or is it the security of having more in the bank? Or?
3. Take the advice a lot of stay at home wives should have considered: Are you willing to bet your material well being on your marriage? What if falls apart before she's 50?
4. Are you willing to bet your family's material well-being on one person's job? What if she gets fired? Or her health breaks down? Would you be able to make up the resources in the time left?
A large part of the reason I want to retire is so that we can spend summers together with our kids(she has off) and go camping for a month or travel, etc.
Could you switch jobs to get that kind of flexibility, even if it meant a pay cut? If this is a school job, could you just get a job at the same place? If you want, you could total up all the summers you've had to work that she hasn't and knock that off your retirement date :)